MacDuff is a character in "Macbeth", and he has a fight with Macbeth, who tells him, "Lay on MacDuff, and damned be him who cries 'Enough!'"
Duff also refers to someone who is not quite good enough at something.
...Though Birnam wood be come to Dunsinane,
And thou opposed, being of no woman born,
Yet I will try the last. Before my body
I throw my warlike shield. Lay on, Macduff,
And damn'd be him that first cries, 'Hold, enough!'...